It's called 'reactance'. A DC you have a near short, but because of the inductance of the windings at 60Hz you have a much higher 'impedance' (not resistance since you are dealing with an inductor). Any introductory electronics text will explain this in detail, right after 'Ohm's law'. R Jai Dhar wrote: > > I guess you are right, there is no substitute for the right part. I do my > shopping at a pretty big surplus store, it's just sometimes they don't have > the exact specs I need. A store down the road usually has everything, > including these transformers, but I have to fork out big $$$ for one since > it's all new stuff (eg. $40 for the xformer I wanted as opposed to $8 at the > surplus store). Anyway, I think I picked up a better one today. While > soldering up the wires, I thought it would be interesting to check the > resistance across the primary and secondary coils. I recall there was some > relationship if you measure the resistance across various terminals in a > xformer... but I couldn't remember. Anyway.. what I found was wierd. Measuring > the resistance across the two wires to the primary coil yielded NO resistance > (ie, close circuit), and the same with the secondary. Is this how it should > be? I did the same with my previous xformer, which was working... and I got > the same results. From what I remember in our intro to Electronics class last > year, xformers are simply just coils side by side (duh)... but with this in > mind, how come there isn't a short since the hot and neutral lines are > connected together??? > > Quoting Russell McMahon : > > > > Sorry to bring this up again, but I was just about to head out to buy my > > > parts... and I started doing some calculations for the resistor that would > > > drop some voltage. First of all, the Vin to my regulators is 25V. Now, > > > straight off, I know this is too high .. I need a better transformer. The > > only > > > problem is if I can find one or not. Right now, I'm going on the > > assumption > > > that I can't. Hopefully I will be home in a few hours with one more > > suitable > > > (that puts out 15V lets say). So anyway, doing the calculations... if I > > want > > > to use these regulators to their full potential (3A and 5A), I will need > > > resistors that are rated at like 25W!!!! Is these a fesable option? Is > > there > > > any other way to drop the voltage?? > > > > As you say, 25b is far too high. If you are in Canada as your email address > > suggests there should be more appropriately rated transformers available. If > > not there are many trade or hobby type internet suppliers. Jameco, Digikey, > > Goldmine, .... > > > > IF you use a linear regulator then that power must be dissipated somewhere. > > If you are dropping form 25v to say 15v then that's 10v to drop. Power = V > > x I = 10 x 3A &r 10 x 5A = 30 watt and 50 watt !!!!!!!!!!! While this > > requires large resistors it would require equally impressive regulators and > > heatsinks without the resistors. What it really means is that, at this power > > level this is almost certainly the wrong approach!!!! > > > > Look in the archives or Google on switching regulators and buck converters. > > Look hard for a lower voltage transformer. Decide if you really need this > > much power at this stage too :-). Note that specifying the transformer > > voltage depends on load and smoothing capacitance. There are lots of > > references on web to power supply design. Google, as always, is your friend. > > As a start, Voutdcpeak = Vac x 1.4. or Vac = Vdc/1,4 (This is due to the > > peak AC voltage being root(2) = 1.414 more than the RMS (average) value. So > > 15/1.4 = 10.7 volts. There will be some droop and ripple so use say 12 VAC - > > a very common transformer value. 12 vac x 1.4 = 16.8Vdc. This will be enough > > to supply 15v as well with a suitable low dropout regulator. Smoothing > > capacitor will need to be large enough to prevent excess ripple. You must > > use a full wave rectifier. There will also be diode drop to account for > > which I have not done above. Again, see Mr Google. > > > > A switching regulator is highly desirable in this case unless there are good > > reasons not to use one BUT I suspect that starting off with less power for > > now may be a good start. If we knew more about your applications we may be > > able to better comment. > > > > > > Russell McMahon > > > > -- > > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > ---------------------------------------- > This mail sent through www.mywaterloo.ca > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body